1. Field of the Invention
The present invention relates to a technique which is applicable to a rolling guide apparatus for guiding a moving object, such as a table, in a machine tool, in particular a guide apparatus of the type that employs finite rollers held by a retainer, and which can effectively prevent displacement of the retainer.
2. Background Art
Precision processing machines conventionally utilize rolling guides for guide of a moving object, such as a table or a saddle. Such rolling guides include the type of guide which uses a plurality of finite rollers held on a flat or V-shaped guide surface by means of a retainer.
A moving object on finite rollers is driven to move by a ball screw. Rolling guides using finite rollers, because of their low friction and high movement precision, are used as guides for ultra-precision feeding.
FIGS. 3 through 5 illustrate a change in the relative positions of retainers with respect to a moving object as observed when the object moves on rolling guides having finite rollers held by the retainers.
In particular, FIG. 3 shows the relative positions of retainers 1, 2 with respect to a table 3, a moving object, in a guide apparatus when the table 3 lies at the middle position of its stroke, and illustrates a case in which there is no displacement of the retainers 1, 2 with respect to the table 3. The retainers 1, 2 are provided on guide surfaces 5, 6 of a bed 4, and lie symmetrical with respect to the table 3 at this moment.
When a ball screw 7 is rotationally driven by a servomotor 8, the table 3, to which is secured a not-shown ball screw nut, moves to the right and left in the Figures. The retainers 1, 2 have the property of moving half the moving distance of the table 3 (for example, the retainers 1, 2 move 100 mm to the left when the table 3 moves 200 mm to the left).
When the movement of the table 3 is repeated, because of slight sliding due to friction with the finite rollers, the retainers 1, 2 become gradually displaced. A one-stroke movement of the table 3 will produce a displacement of e.g. about several μm. FIG. 4 illustrates a case in which the retainer 1 has been displaced to the right.
Such displacement gradually increases as the movement of the table 3 is repeated and, when the table 3 has a long stroke, it is possible that the table 3 may partly slide off the retainer 1 or 2, which could cause an accident. When the table 3 is heavy there is a case where the retainers 1, 2 do not slide between the bed 4 and the table 3, leading to buckling and breakage of the retainers 1, 2. The repair of a retainer needs a considerable time.
It is, therefore, a conventional practice to provide sensors 9a, 9b, 9c and 9d on both sides of the table 3 to detect partial sliding off of the table 3 from the retainer 1 or 2 and to carry out a position correction for a displaced retainer as shown in FIG. 5.
FIG. 5 is a top plan illustrating correction of the position of the retainer 1 carried out by moving the table 3 to the right and bringing the retainer 1 into contact with a stopper 10. As shown in FIG. 5, when the table 3 is moved to the right, the displaced retainer 1 comes into contact with the stopper 10 provided at the right end of the guide surface 5. A total of four stoppers 10 are provided at the longitudinal ends of the guide surfaces 5, 6. When the table 3 is further moved to the right with the retainer 1 kept in contact with the stopper 10, the retainer slides on the table 3 whereby the displacement is corrected.
The displacement correction method, which involves causing the displaced retainer 1 or 2 to collide against the stopper 10, may cause deformation of the retainer, thus adversely affecting the positioning accuracy, straightness, etc. of the machine.
Further, in view of the gradual increase in the displacement of retainers 1, 2 with the repetition of the movement of the table 3, it is a conventional practice to periodically carry out the position correction for the retainer 1 or 2 by contact of the retainer with the stopper 10, which requires needless wasted time. In addition, displacement of the retainers 1, 2 does not occur in a constant direction. This necessitates a troublesome operation of detecting a direction, in which displacement is to be corrected, with the sensors 9a, 9b, 9c and 9d, and determining the moving direction of the table 3 in carrying out the position correction of the displaced retainer 1 or 2.
In view of the above, the applicant has proposed a technique which prevents displacement of a retainer by making use of its property of moving half the moving distance of a table when the table moves (Japanese Patent Laid-Open Publication No. 2004-116592). The technique involves using a ball screw in addition to a ball screw for feeding of the table, reducing the rotating speed of the table-feeding ball screw to one-half and transmitting the torque to the other ball screw, and moving a case, to which a stopper for preventing displacement of the retainer is mounted, by means of the reduced-speed ball crew.